Stretched films made by stretching a resin are used as optical films in various display devices for serving various optical functions, by taking advantage of optical anisotropies thereof. For example, it is known to use stretched films in a liquid crystal display device as an optical compensation film for optical compensation such as coloring prevention and viewing angle expansion, or as a phase difference film that also performs as a polarizer protection film by having a polarizer laminated to the stretched film.
In recent years, self-luminous display devices such as an organic electroluminescence display device have been gathering attention as a new display device. Self-luminous display devices have more potential for minimizing power consumption as compared to liquid crystal display devices whose backlights are constantly on. Furthermore, self-luminous display devices which light separate light sources for each color as like the organic electroluminescence display device require no color filter that is a cause for the decrease in contrast. As a result, it is possible to further improve the contrast.
Organic electroluminescence display devices are provided with a reflective body such as an aluminum plate on the back side of the display to improve light-extraction efficiency. Unfortunately, light entered from outside into the display and reflected on this reflective body causes degradation in the contrast of the image. For the prevention of the reflection of external light to improve the contrast in brightness, there is known a technique in which the stretched film is laminated to a polarizer and a circular polarizer is used on the front side of the display.
Such a circular polarizer requires to be laminated in a position in such a way that an in-plane slow axis of the stretched film is inclined in a desired angle with respect to the transmission axis of the polarizer.
A general polarizer (polarizing film) however is achieved by stretching the film at a high ratio along a course on which the film runs, whose absorption axis runs in the same direction as the running course of the film, and a currently known phase difference film is produced by lateral or transverse stretching; in principle, the in-plane slow axis extends in a 0° or 90° direction with respect to the longitudinal direction of the film. Therefore, the only way to achieve a desired inclination angle between the absorption axis of the polarizer and the slow axis of the stretched film is to perform batch processing, which is a process cutting out a long polarizing film and/or the stretched film at a specific angle and laminating the cut film strips one piece each. This causes issues such as the decrease in productivity due to the misalignment in the slow axis and the decrease in yield due to the adhering of chips and like matter on the films.
Patent Literature 1 proposes a method for producing a long phase difference film in which a phase difference film is stretched in a desired oblique angle and a slow axis thereof is freely controllable in a direction neither in the 0° nor 90° angles with respect to the longitudinal direction of the film. This method laminates the polarizing film and the stretched film by a roll-to-roll process. As a result, no decrease was found in yield, and thus allowed stable production.
However, the oblique stretching method disclosed in Patent Literature 1 varies in film thickness and optical characteristics, between plural widthwise positions of the film. For example, in a case of producing a film having a large alignment angle, the variation in the widthwise direction of the film markedly appeared. In order to solve such issues, Patent Literature 2 discloses a method of matching an angle between an alignment direction of the stretched film and partition walls separating sections in a heating furnace. Moreover, Patent Literature 3 discloses a production method which produces a film in such a way that angles between the alignment direction of the stretched film and boundaries of the stretching, thermally fixing, and cooling sections are within a constant specific range, and an absolute value of the differences between each of these angles meet within a specific range.
However, in the conventional production methods, a variation occurs in optical alignment on the film ends in a case in which the film is stretched obliquely while the film is run at fast running velocity, thereby causing a large alignment unevenness throughout the entire film. Moreover, if this film with such an alignment unevenness is used as the circular polarizer, partial color irregularity would appear on the circular polarizer, thereby causing a marked drop in the quality of the circular polarizer.
Moreover, Patent Literature 4 proposes a circular polarizer prepared by stacking and laminating a λ/2 phase difference film and a λ/4 phase difference film in such a way that the in-plane slow axes thereof form a preferred angle, so that a circular polarizer is obtained that has a high degree of circular polarization across a broad visible light wavelength range (broadband circular polarizer). In order to prepare such a film by the roll-to-roll process, each of the in-plane slow axis of the films need to be inclined in different angles, with respect to the widthwise direction.
On this account, instead of an oblique stretching device that can only stretch the film in a specific angle, it is preferable to use an oblique stretching device that is capable of stretching the film at any preferred angle by changing a stretching pattern in the single oblique stretching device and preparing a long stretched film having any preferred inclining angle of the in-plane slow axis.
In comparison, the above conventional production method shows significant variation in optical alignment of the film when the stretching pattern is changed. Such a film causes a large color irregularity when used in the circular polarizer. As a result, products could not be produced.